1. Field of the Invention
The present invention relates to a radio apparatus configured to control variable communication rate. In particular, the present invention relates to a radio apparatus which controls the rate such that the communication rate is varied in accordance with variation of the environment.
2. Description of the Related Art
Recently, cdma2000 1x-EV DO (hereinafter referred to as EV-DO) has been developed as a next-generation high-rate wireless communication system. The EV-DO system is a new version, which is specialized for the purpose of data communication and is higher in transmission rate, compared to the cdma2000 1x system, which is an extended version (third generation system) of the cdmaOne system. “EV” stands for “evolution”, and “DO” stands for “data only”.
The EV-DO system is approximately the same as the cdma2000 1x system in the configuration of a radio interface of an upstream channel from a radio communication terminal to a base station. As for the configuration of the radio interface of a downstream channel from a base station to a radio communication terminal, whereas the bandwidth of 1.23 MHz is the same with that of the cdma2000 1x system, the modulation method and the multiplexing method are much different from those of the cdma2000 1x system. As for the modulation method, whereas QPSK and HPSK are used in the cdma2000 1x system, QPSK, 8-PSK, or 16QAM is selectively used in the EV-DO system according to a reception state of a downstream channel of a radio communication terminal. As a result, a high transmission rate with low error resistance is used when the reception state is good, and a low transmission rate with high error resistance is used when the reception state is bad.
As for the multiplexing method for enabling multiple communications from one base station to a plurality of radio communication terminals, TDMA (Time Division Multiple Access) is employed in EV-DO system. In such system, communications with a plurality of radio communication terminals are performed in such a manner that time is divided in units of 1/600 second, a communication is performed with one radio communication terminal in each unit time, and the radio communication terminal to be communicated is switched every unit time, rather than CDMA (Code Division Multiple Access) which is employed in the cdmaOne system and the cdma2000 1x system.
A radio communication terminal measures a carrier-to-interference power ratio (hereinafter abbreviated as CIR) of a pilot signal as an index of a reception state of a downstream channel from a base station to be communicated, predicts a reception state during the next reception time slot on the basis of a variation of the CIR, and notify “a maximum transmission rate which enables to receive with a error ratio that is lower than a predetermined rate”, which is expected from the predicted reception state to the base station as data rate control bits (hereinafter referred to as DRC) predetermined. The predetermined error rate is usually set to about 1% though it depends on the system design. The base station receives the DRCs from a plurality of radio communication terminals, and a scheduler function in the base station determines with which radio communication terminal is to communicate in each division unit time. Basically, as high a transmission rate as possible is decided on the basis of a DRC sent from each radio communication terminal and is used for a communication with it.
With the above configuration, the EV-DO system enables a maximum transmission rate of 2.4 Mbps (mega-bits per second) per sector in a downstream channel. This transmission rate is the sum of amounts of data communications from one base station to a plurality of radio communication terminals in one frequency band and in one of a plurality of sectors (usually, a plurality of sectors exist). The transmission rate increases if a plurality of frequency bands are used.
JP-A-2002-300644 is known as a related art.
As described above, in the EV-DO system, the transmission rate of a downstream channel depends on the reception state of a radio communication terminal. Although the reception state is as high as 2.4 Mbps in the best reception state in a stationary state, it is about 500 to 700 kbps on average in a state that a radio communication terminal is moving at a middle or high speed with a vehicle and is as low as tens of kilo-bits per second in a bad reception state in a stationary state. Therefore, even when the user of a radio communication terminal is moving at a low speed (i.e., walking) or almost stands still, the transmission rate depends on the location and may decrease to a large extent. To prevent the condition by user's operation so-called antenna mark is displayed and an alarm sound is generated in order to notify the user of the reception state in conventional cellular phones. For example, cellular phones which uses cdmaOne system notify users of a reception state on the basis of Ec/Ic (energy per chip to the total input power).
However, in the EV-DO system, the transmission rate of a downstream channel is influenced by not only the CIR instantaneous value but also a correction using statistical data such as predictive or past data transmission error rates in downstream channels. Therefore, an error may be contained under a reception state decided on the basis of only a CIR. Further, in EV-DO cellular phones, the variation of the transmission rate due to a reception state variation is larger than that in PDC (Personal Digital Cellular) and cdmaOne cellular phones. Therefore, in the EV-DO system, it is required that a reception state is measured with higher accuracy.
On the other hand, in the EV-DO system, the upstream channel transmission power of a radio communication terminal is controlled by a base station as is the case with the cdma2000 1x system. The maximum transmission power is limited to +23 dBm (200 mW) to +24 dBm (about 250 mW) by regulations etc. A base station instructs, whenever necessary, each radio communication station to increase or decrease the transmission power so that the transmission power of each radio communication station is kept almost constant or satisfies desired quality. According to such an instruction, each radio communication station adjusts the transmission power in a range below the above-mentioned maximum transmission power. When a radio communication terminal is located away from a base station and it is hard for an upstream channel signal to reach the base station, the base station instructs the radio communication terminal to increase the transmission power. However, if the transmission power of the radio communication station has reached the maximum transmission power, the radio communication station cannot increase the transmission power any more. The DRC of the upstream channel does not reach the base station. As a result, downstream channel data transmission can no longer be performed.
In radio communication terminals, a demodulator has an AGC (automatic gain control). The transmission rate depends on only the CIR, and is not generally influenced by the reception signal power in an AGC operating range. On the other hand, if the reception signal power becomes lower than the AGC operating range, the CIR steeply decreases and a downstream channel signal can no longer be received.
The transmission rate per sector is shared by all the radio communication terminals that exist in the sector and are connected to the base station. Therefore, the transmission rate that can be used by each radio base station is varied to a large extent by the communication states according to the other radio communication terminals. When a plurality of radio base stations exist in one sector and are communicating with the base station, the allocation of downstream channel transmission rates of the respective radio communication terminals is determined by a scheduler in the base station. Although no scheduler algorithm is defined in the standard, an algorithm called “proportional fair” is generally considered effective in terms of the fairness of provision of transmission rates to respective radio communication terminals and the improvement of the sector transmission rate.
According to the proportional fair algorithm, DRC/R is calculated on the basis of amounts R of data that have just been sent from the base station to the respective radio communication terminals in 1.67 s (corresponds to 1,000 slots) and DRC values that are requested by the respective radio communication terminals. A slot is allocated to a radio communication terminal having a largest DRC/R value. With this scheduler processing, an actual transmission rate is not necessarily equal to a transmission rate that a radio communication terminal requests the base station to allocate using a DRC. In general, the actual transmission rate is lower than the transmission rate that a radio communication terminal requests using the DRC. Therefore, if a DRC itself is used as an index of the reception state, it is insufficient because the presence of the other radio communication terminals is not taken into consideration.